refactor(model): transformers style model call

[LongxingTan]

Description

This PR ...

Checklist

• Linked issues (if existing)
• Amended changelog.md for large changes (and added myself there as contributor)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

Thank you for joining. Have fun coding!

[codecov]

Codecov Report

Attention: Patch coverage is 84.01487% with 43 lines in your changes missing coverage. Please review.

Project coverage is 84.85%. Comparing base (b7658d2) to head (0a4d26b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
tfts/trainer.py	79.13%	29 Missing ⚠️
tfts/models/transformer.py	82.05%	7 Missing ⚠️
tfts/layers/embed_layer.py	62.50%	6 Missing ⚠️
tfts/layers/position_layer.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   85.53%   84.85%   -0.69%     
==========================================
  Files          33       33              
  Lines        2627     2575      -52     
==========================================
- Hits         2247     2185      -62     
- Misses        380      390      +10     

Flag	Coverage Δ
cpu	84.85% <84.01%> (-0.69%)	⬇️
unittest	84.85% <84.01%> (-0.69%)	⬇️

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